The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Physical vapor deposition (“PVD”) systems deposit metal layers onto substrates such as semiconductor wafers. The metal layers can be used as diffusion barriers, adhesion or seed layers, primary conductors, antireflection coatings, etch stops, etc.
In PVD systems, plasma feed gas such as Argon is introduced into a chamber. Electrons collide with atoms of the plasma feed gas to create ions. Magnetic fields are used to increase a residence time of the electrons by causing the electrons to spiral through the plasma. As a result, ionization levels of the plasma feed gas also increase. As the number of ions increase, the deposition rate also tends to increase.
A negative potential applied to a cathode attracts the ions towards a target. The ions collide with the target with high energy. Target atoms are dislodged from the surface of the target by direct momentum transfer. The impact of the ions on the target also releases secondary electrons. The dislodged atoms and ions (electrostatically attracted by the secondary electrons) are then deposited on a substrate such as a semiconductor wafer.
Some applications require the deposition of a thin layer of barrier or liner metal in a trench or via. Deposition of sufficient material onto a bottom or sidewall depends on the capability of the PVD process to direct the flow of sputtered atoms onto the substrate. In gap-fill applications, or filling of vias and trenches with primary metals, obtaining acceptable step coverage similarly requires directionality of sputtered atoms.
Some PVD systems use additional electromagnetic coils in the transfer region to adjust and shape the magnetic fields. The PVD systems may create one or more magnetic null points to provide a directed beam of ions emanating from the target region. The target region and the transfer region are magnetically separated by a separatrix, a surface upon which magnetic field lines converge and on which the null point(s) reside. Since the PVD systems generally use concentric electromagnetic coils to generate the magnetic field, the null point is usually situated on the central axis of the PVD system. As a result, a deposition profile of these PVD systems usually has a large radial gradient.
Maximizing ion collection below the separatrix and spreading the ions uniformly across the substrate are typically opposing goals. In a PVD system with concentric electromagnetic coils and a single null point, maximizing ion collection usually involves maximizing the coil currents to maximize ion confinement. This usually leads to a very narrow beam of ions and relatively poor center to edge uniformity at the substrate.
Attempting to spread the ion beam while maintaining high ion density by lowering the confinement field and/or imposing a counter field near the wafer level to diverge the field lines just above the wafer has proven to be difficult.